The present invention relates generally to foam-in-place structural materials used for reinforcement of structural members. More particularly, the present invention relates to a two-component epoxy/amine foamed material exhibiting improved mechanical properties (good balance of high compressive strength, compressive modulus, glass transition temperature and cured ductility) as well as enhanced shear-thinning characteristics.
Traditional foam-in-place structural materials known in the art generally disclose polyurethane materials, polyurea, or epoxy-based materials. These materials incorporate a method to create volumetric expansion and a curing mechanism as well to effectuate curing at room temperature and achieve a degree of control of expansion and cure rate characteristics. Although these prior art materials are both useful and successful in a number of applications, certain structural reinforcement applications in the automotive industry, for example, would benefit from a material having an improved balance of mechanical properties, such as a higher compressive strength, little change in modulus over a broad temperature range and a glass transition temperature that exceeds 200xc2x0 F. In addition, improved cured ductility that enables the material to deform plastically when stresses exceeding the material yield strength are applied would provide definite benefit. Further, these structural reinforcement characteristics in many applications, including automotive, may also benefit from a shear-thinning structural material which exhibits an increased viscosity at zero shear rate and a decreased viscosity at higher shear rates prior to curing. This enables the material to flow more easily while being dispensed but then have flow minimally following dispensing. This shear thinning behavior can also assist with the development of a uniform, consistent foamed cell structure by allowing more effective foaming gas entrapment.
As known by those skilled in the art, a number of factors determine the suitability of a process for forming a foamed product of the type in which a blowing agent forms cells in a synthetic resin as the resin is cured. Most significantly, the interaction of the rate of cure and the rate at which the blowing gas is generated must be matched to create the proper cured product. If the resin cures too rapidly there is inadequate time for the gas to form the proper size and number of gas voids in the finished product. Over expansion of the forming foam product must also be avoided. Rapid expansion due to a slow cure rate relative to gas evolution may cause the expanding foam to simply collapse as a result of inadequate wall strength surrounding the individual gas cells.
A number of prior art techniques are available to control the rate of foam expansion and the cure rate. For example, a wide-range of reactivities are available in commercial resins and curing agents. In addition, resins are available in a range of viscosities, which is another parameter and can be used to control the foam expansion rate. That is, it is known that a low viscosity resin can generally be expanded to a greater volume with a given volume of gas than a higher viscosity material; however, the resin must have sufficient viscosity to contain the gas at the pressures at which it is generated in order for the foam to be properly formed.
With respect to automotive applications, foamed products must have good environmental resistance and, most significantly, in many applications they must protect metal from corrosion while maintaining adhesion to the substrate. In the past many foamed parts were made using polyurethane, which provides a number of desirable attributes. It is known, however, that alternatives to urethane-based foams or more precisely materials based on the reaction of the isocyanate chemical functional group are frequently more environmentally desirable, in part due to the potential for unreacted functional groups in the finished products and difficulty in handling isocyanate functional chemicals in manufacturing processes. In addition, the polyurethane materials found in the prior art fail to provide optimum mechanical properties, generally possessing lower elastic modulus strength and lower glass transition temperature than what is capable with epoxy-based materials. In comparison with polyurethane materials, however, the epoxy-based materials found in the prior art often exhibit both poor cured ductility and higher viscosity during dispensing.
Accordingly, there is a need in industry and manufacturing operations for a structural material, which exhibits improved mechanical properties, such as higher compressive strength, compressive modulus, and glass transition temperature, as well as better-cured ductility. The improved mechanical properties allow the structural material of the present invention to be capable of plastically deforming when loaded beyond its yield stress. However, unlike prior art materials, there is no significant reduction in modulus or glass transition temperature. In addition, there is a need for an improved material, which can be used in a variety of applications wherein one or both components utilize a thixotropic filler, which produces pronounced shear-thinning characteristics. By providing a material with excellent cured physical properties and desirable processing attributes, the present invention addresses and overcomes the shortcomings found in the prior art.
The present invention relates to methods, materials, and products for foam-in-place structural reinforcement of hollow structures such as automobile cavities. In one embodiment, the present invention comprises a two-component foam-in-place structural material for producing a foamed product. Though other resin systems are possible, the first component of the system includes an epoxy-based resin. Preferably, the first component is formulated with a physical blowing agent, and more preferably one having a shell or skin that will change state to provide volumetric increase to create expansion. For example, the shell is a thermoplastic that, upon heating, will melt or soften to enable a solvent core to expand the shell. The second component includes an amine, and is formulated with an agent for allowing the resulting material to exhibit ductility with little reduction in modulus, glass transition temperature, or both. It is contemplated that the amine of the present invention could be a primary or secondary amine. Generally speaking, the amine is an epoxy curing agent or modifier, and preferably, a high solids epoxy curing agent, though it could be a water-borne epoxy-curing agent. Other examples of an amine suitable for use in the present invention include polyamides, aliphatic amines, and cycloaliphatic amines as well as other agents that can function as accelerators or catalysts. An optional thixotropic filler is included in either or both of the first or second components, and possibly as a stand-alone component. In one embodiment, this additive preferably causes the material to have high viscosity at a near zero shear rate and low viscosity at a higher shear rate, which is more commonly known in the art as shear-thinning.
The present invention provides a method of forming a foamed product, which comprises the steps of combining the first component (with a blowing agent) with the second component (with a curing agent). The first component, preferably an epoxy, is cross-linked through a polymerization reaction with the second component of the formulation (e.g. an amine). In this regard, an exothermic reaction or reactive mixture is created between the-epoxy component and the amine component when combined. The heat generated by the exothermic reaction softens the thermoplastic shell of the blowing agent formulated within the epoxy component thereby enables the solvent core within the thermoplastic shell to expand the thermoplastic shell and thereby create expansion. In a preferred embodiment the mixture of materials is in liquid form. However, it is contemplated that the mixture of materials could also comprise a paste or solids of varying viscosities and textures.